2027 ICD-10-CM Diagnosis Code G91.1Obstructive hydrocephalus
ICD-10-CM Codes›G00–G99›G89-G99›G91
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 127
- Chronic Condition
G91.1 is a billable ICD-10-CM diagnosis code for obstructive hydrocephalus. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 56 through 57. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 108 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (neither hereditary nor degenerative).
For Medicare Advantage risk adjustment, G91.1 maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified) under the V28 model, adding a risk factor of about 0.341 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
G91.1 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acquired obstructive hydrocephalus
- Dementia due to obstructive hydrocephalus
- Hydrocephalus associated with late onset aqueduct stenosis
- Hydrocephalus due to and following meningitis
- Hydrocephalus due to intracranial neoplasm
- Hydrocephalus due to obstruction of fourth ventricle outflow
- Hydrocephalus following traumatic injury
- Obstructive hydrocephalus
- Obstructive hydrocephalus due to and following meningitis
- Obstructive hydrocephalus due to entrapment of inferior horn of lateral ventricle
- Post-traumatic non-communicating hydrocephalus
- Sequelae of meningitis
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to G91.1: its own notes plus those printed at G91 and Chapter 6. A note printed at a category, block or chapter applies to every code under it.
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
- certain conditions originating in the perinatal period (P04-P96)
- certain infectious and parasitic diseases (A00-B99)
- complications of pregnancy, childbirth and the puerperium (O00-O9A)
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
- endocrine, nutritional and metabolic diseases (E00-E88)
- injury, poisoning and certain other consequences of external causes (S00-T88)
- neoplasms (C00-D49)
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name G91.1, its category, or a range that includes it.
Excludes1 2
These codes carry an Excludes1 note naming G91.1: they are not reported together with it, unless the two conditions are unrelated.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Hydrocephalus (acquired) (external) (internal) (malignant) (recurrent) G91.9
noncommunicating G91.1
obstructive G91.1
Stenosis, stenotic (cicatricial) See Also: Stricture;
Stricture See Also: Stenosis;
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Non-Communicating Hydrocephalus
an abnormal accumulation of cerebrospinal fluid within the ventricles of the brain that occurs as a consequence of an obstruction at any location within the ventricular system that prevents cerebrospinal fluid flowing into the subarachnoid space.Hydrocephalus
excessive accumulation of cerebrospinal fluid within the cranium which may be associated with dilation of cerebral ventricles, intracranial hypertension; headache; lethargy; urinary incontinence; and ataxia.
Patient EducationClinical
Hydrocephalus
Hydrocephalus is the buildup of too much cerebrospinal fluid in the brain. Normally, this fluid cushions your brain. When you have too much, though, it puts harmful pressure on your brain.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert G91.1 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About G91.1Overview
What is the ICD-10 code for obstructive hydrocephalus?
The ICD-10-CM code for obstructive hydrocephalus is G91.1 (sometimes written as G911). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is G91.1 (Hydrocephalus) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report obstructive hydrocephalus on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does G91.1 group to?
When obstructive hydrocephalus is the principal diagnosis on an inpatient stay, it groups to MS-DRG 57, 56, with relative weights from 1.2835 to 2.2649 depending on complications. Higher weights mean higher Medicare reimbursement.
Is G91.1 a CC or MCC?
CMS lists G91.1 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 108 closely related codes in its exclusion list.
Can G91.1 be reported with a code from Q07?
Not as a rule. The Excludes1 note at G91, which applies to G91.1, lists "Arnold-Chiari syndrome with hydrocephalus (Q07.-)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
What HCC is G91.1?
G91.1 (obstructive hydrocephalus) maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified), commonly written as HCC 127, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 51 under the retired V24 model. It also maps in the ESRD (V21) and ESRD (V24) models. It does not map to any RxHCC in the Part D prescription drug model.
Does G91.1 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, G91.1 adds a risk adjustment factor of about 0.341 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.000 to 0.438 depending on the payment segment). A more severe related category (HCC 125 and HCC 126) supersedes it when both are reported. See the full factor table on the HCC 127 category page.
